The invention relates to a power capacitor cell, with a metallized film, essentially used for correcting the power factor in low voltage. The capacitor is called self-healing, for the presence of minor faults in the dielectric film results in a localized puncture, leading to evaporation of the electrodes in the vicinity of the fault without damaging the adjacent film. A capacitor cell is achieved by winding metallized plastic films and the capacitor consists of an assembly of wound cells, connected according to a parallel series arrangement, depending on the power. When abnormal stresses occur, notably overvoltages or temperature rises, a dielectric breakdown may damage the self-healing properties of the capacitor. The resulting increase of the current flowing through the cell causes the temperature to rise in the vicinity of the fault and decomposition gases of the plastic film to be produced. The effect of the pressure may cause the casing to explode. A conventional protective device consists in placing the cell components in a metal box sufficiently solid to contain the gases emitted. The fault is cleared from the circuit after the cell affected by the fault has been completely destroyed, but there remains a high internal short-circuit risk and a risk of explosion. Another state-ofthe-art solution consists in providing each capacitor cell with an electrical fuse which interrupts the circuit and disconnects the faulty cell when an overcurrent occurs. This protection is ineffective when the fault is impedant, therefore when the short-circuit current is limited.
The pressure increase causes deformation of the casing and it has already been proposed to fit a cover which can be deformed or ejected by the effect of the pressure, this deformation or ejection tearing a cell connecting wire off so as to disconnect it. This protection is effective for small overcurrents, but has the drawback of restriking when disconnection takes place with high short-circuit currents. Such a restriking causes the casing to open or to explode with a risk of fire or at least a large amount of fumes given off damaging the neighbouring equipment. An alternative described in this document consists in cutting the insulation of a connecting wire to create a short-circuit, but the operation is not reliable.